docs: def split_kwargs(relaxation_kwds):
      """Split relaxation keywords to keywords for optimizer and others"""
      optimizer_keys_list = [
          'step_method',
          'linesearch',
          'eta_max',
          'eta',
          'm',
          'linesearch_first'
      ]
      optimizer_kwargs = { k:relaxation_kwds.pop(k) for k in optimizer_keys_list if k in relaxation_kwds }
      if 'm' in optimizer_kwargs:
          optimizer_kwargs['momentum'] = optimizer_kwargs.pop('m')
      return optimizer_kwargs, relaxation_kwds